As shown in FIG. 2, such a ribbon microphone is provided with an acoustic-electric converter (i.e. microphone unit) 1. The converter 1 has a metallic foil such as an aluminum foil in the form of a strip of several micrometers thick as a diaphragm 10. The foil is placed in a parallel magnetic field formed by a pair of permanent magnets 30 facing each other with a predetermined space therebetween. Attachment electrodes 20, which include a pair of support electrode plates 20a and 20b, are attached to opposite ends 10a and 10b of the diaphragm 10.
Such a ribbon microphone is bi-directional and mass controlled. This significantly lowers the resonance frequency and allows sounds to be collected in a lower tone range.
A problem with a ribbon microphone is that, on impact against the microphone, an inertial force of the diaphragm (sometimes referred to as “ribbon”) 10 stretches the ribbon foil, leaving it in an elongated state by plastic deformation. When a ribbon plastic-deformed in this way contacts a magnetic pole or a nearby component, the performance may significantly be degraded.
For this reason, it is a common practice to provide protections against shocks during transportation such as cushioning materials attached to the inside and/or outside of a box containing a microphone so as to avoid a direct impact on the microphone.
Besides shocks during transportation, however, other shocks on impact against a microphone include a drop impact experienced, for example, when the microphone is accidentally dropped while being handled on a microphone stand for attachment. It is therefore necessary to protect a ribbon against shocks also when the microphone is not in use (i.e. when the microphone is deactivated) except for transportation.
Thus, the applicant has proposed in Japanese Patent Application Publication No. 2009-218685 to suppress vibrations of a ribbon by electromagnetic damping when the microphone is not in use.
The arrangement will now be described briefly. There is provided a mechanical switch that turns on (i.e. closed), for example, when a plug provided on an end of a cable (i.e. cable end plug) on the phantom power supply side is not plugged into an output connector of ribbon microphone and turns off (i.e. open) once the cable end plug is plugged. The switch is turned on and off across the ribbon.
In this way, when the cable end plug on the power supply side is not plugged into the output connector and the microphone is not in use, the switch turns on to create an electrically short circuit across the ribbon, resulting in a closed circuit including the ribbon.
In this state, if the ribbon is moved within the parallel magnetic field (i.e. magnetic gap) on impact against the microphone, a back electromotive force is generated in the ribbon. The back electromotive force causes a current to flow through the closed circuit to generate an electromagnetic braking force. Since the braking force acts in the direction opposite to the direction of vibration of the ribbon, the vibrations of the ribbon can be suppressed.
As described above, according to the invention set forth in Japanese Patent Application Publication No. 2009-218685, when the cable end plug on the power supply side is not plugged into the output connector during transportation or while the microphone is being handled for installation, the movement of the ribbon is restricted by electromagnetic damping even upon impact against the microphone. Thus, the elongation, along with plastic deformation, of the ribbon can be prevented.
In the invention set forth in Japanese Patent Application Publication No. 2009-218685, however, a mechanical switch, which is turned on and off by the cable end plug, is provided on the output connector. Therefore, a custom-designed, special output connector is required.
An object of the invention, therefore, is to provide electromagnetic damping without a special output connector, in order to protect a diaphragm of metallic ribbon foil of a ribbon microphone against shocks.